To study a.c. wave forms using a cathode ray oscilloscope In addition to the tube, a cathode ray oscilloscope contains a sweep generator or time-base circuit. When this is switched on it applies a potential difference to the Xplates which builds up uniformly with time to a maximum and then repeats the process at regular intervals. The result […]
The cathode ray oscilloscope The cathode ray oscilloscope is an instrument used for studying the current and voltage wave forms in various electric circuits. In this connection it is very useful for checking laboratory electric equipment and television and radio receivers. The chief feature of an oscilloscope is the cathode ray tube, which is a vacuum tube containing […]
Experimental thermionic electron tubes Fig. 44.11 shows a thermionic tube designed to show the deflection of moving electrons in an electric field. Its electron gun consists of a cylindrical cathode with a hot metal filament inside to provide a supply of electrons by thermionic emission. The electrons are ac celerated towards a cylindrical anode and emerge as a […]
What happens inside an electric discharge tube? We shall now return to the electric discharge tubes described earlier in this chapter. The processes which go on inside these tubes are complex and difficult to investigate, so that physicists hold different views regarding explanations of the various dark spaces, striations and so on. However, all agree that ions […]
The Edison effect. Thermionic emission In 1883, the American scientist and inventor, Thomas Edison, was experimenting with an incandescent electric filament lamp which also contained a small metal plate supported by a wire sealed through the glass. When a battery and galvanometer were connected between the plate and filament he found that a small current flowed through the galvanometer […]
Applications of X-rays The X-ray photographs or radiographs used in medical practice are made by allowing X-rays to pass through parts of the body and on to a photographic film. X-rays are used in hospitals also in the treatment of malignant growths, as it is found that cancer cells can be destroyed by this means. Nevertheless, great care […]
The Coolidge hot-cathode tube For reasons which cannot be discussed here the wavelength of the X-rays produced by the early gas tubes was difficult to control. Modern tubes are based on a method devised by W. D. Coolidge in 1916 (Fig. 44.8). In this tube the cathode takes the form of a spiral of tungsten wire which, […]
The gas-filled X-ray tube Later investigation showed that X-rays were produced whenever fast moving electrons were stopped on impact with a target. This led to the development of tubes containing air at very low pressure in which a beam of cathode rays was focused from a concave cathode on to a tungsten disc called the anticathode (Fig. 44.7). […]
The discovery of X-rays In December 1895 the German physicist Wilhelm Rontgen noticed that some barium platinocyanide crystals glowed brightly in the neighbourhood of a working cathode ray tube even when the tube was covered up. He also noticed that some wrapped photographic plates left near the tube had become fogged. It appeared that some kind of invisible radiation […]
Specific charge The ratio e/m2 mentioned in the previous paragraph is called the specific charge of an electron. Its units are coulmbs/kilometer. Note the use of the word specific which, as we have seen before in this book, refers to a property of unit mass of something.